The present invention relates to a system for reproducing a reference position in an incremental path measuring system of the type which includes an incremental measuring graduation provided with at least one reference mark secured to one of the objects, the position of which is to be measured, and a scanning arrangement secured to the other of the objects, the position of which is to be measured.
Systems for determining reference positions in incremental measuring devices are known to the art, in which machine or measuring system components which are movable relative to one another are moved from a starting position to a reference position. The distance traveled between the starting position and the reference position is measured and stored, or the reference position defined by a reference mark is used to determine a position, such as a zero position in a measuring system. Such a process can be used, for example, in an incremental length or angle measuring system such as that described in German patent DE-PS No. 19 64 381. This process requires, however, an unimpeded relative movability of the objects to be measured, since the components of the measuring arrangement are firmly connected with the objects to be measured and the objects to be measured must be shifted in unison with the components of the measuring system up to the position defined by the reference mark.
German DE-OS No. 16 73 887 discloses a prior art measuring system in conjunction with a machine which includes a slide piece and a bed. The disclosed measuring system can be used to determine a reference position when the slide piece is clamped fast to the machine bed. As a first step of the disclosed system, the slide piece is driven into the position which is later to be taken as the reference or zero position. The slide piece is then clamped fast to the machine bed and the scanning plate is then moved relatively to the scale until a reference mark is detected. On reaching the reference mark, the electronic counter of the measuring system is set to zero. The clamping for the machine parts can then be released and the slide piece moved into its desired position. The position of the reference mark, therefore, represents the reference position for further operating cycles.
These prior art methods for determining a reference value for a starting position (which occurs before the beginning of the working cycle proper) cannot in general be used with an incremental measuring system if the working cycle has already commenced. For example, such methods are not as a general rule suitable for use in working cycles that are interrupted while in progress. Such interruptions of a running working cycle can occur, for example, in the case of an automatic handler such as an industrial robot in the event of a power failure. In this case the robot will remain at a standstill at the momentary position it occupied at the instant of power failure. As a result of the power failure, the measuring value determined previously according to the methods described above and referred to the original reference position is lost, since the measurement is interrupted.
In order to continue an interrupted working cycle, the reference position must be redetermined in some manner. A return movement of the robot out of its momentary position into the original starting position is not possible as a general rule. For example, a tool held by the robot may happen to be in engagement with a workpiece at the time of the power interruption, thereby preventing relative movement between the tool and the workpiece.